Until recently displacement sensors such as accelerometers has been based on capacitor structures and impedance measurements. This has a number of disadvantages related to sensitivity, high voltage biasing, isolation between layers, alignment and positioning of membrane relatively to back electrode, high requirements to preamplifiers, and nonlinear response, all resulting in costly and complicated solutions.
Optical position encoders are able to detect the lateral displacement of a scale relative to a reading head. The reading head includes a light source illuminating a reflective diffraction grating patterned on the scale. The diffraction grating acts as a beam splitter which, associated with other optical components, produces interference fringes on the reading head. These interference fringes moves with the scale and their position can be measured by means of one or several detectors. There are several implementations of such encoders, differing especially in the way the different beams illuminating and diffracted by the grating on the scale are combined to produce interference fringes, such as [1], [2] and [3].
With the aim to integrate a miniaturized position sensor with MEMS devices, another type of grating-based position sensor was invented. In this prior art, the grating consists of a first surface with reflective lines and a reflective unpatterned second surface below. The whole structure can be analysed as a deformable diffractive structure with grooves consisting of two levels: a top level consisting of reflective lines on the first surface and a bottom level consisting of the portion of the second surface underneath the area between the reflective lines of the first surface. As the distance between the two surfaces changes, the height of the grooves of the diffractive structure changes, with the effect of changing its diffraction efficiency. [4], [5].
The way to operate such a position sensor is quite different from [1,2 & 3]. In this case the grating on the first surface is in a fixed position relative to a reading head placed above the first surface. The illumination is not designed as to produce interference fringes with the grating, but rather to be able to distinguish at least one diffraction order reflected by the grating. One or several photodetectors on the reading head are used to measure light intensity in one or several of these diffraction orders. Note that this diffraction orders are at fixed position on the reading head, as the grating on the first surface is not moving compared to the reading head. But as the diffraction efficiency of the diffractive structure is modulated, the portion of light diffracted to the different diffraction orders is changed. To summarize, a change in the distance between the two surfaces changes the height of the grooves making a deformable diffractive structure, which in turn changes the diffraction efficiency of the diffractive structure, which can be measured by photodetectors placed accordingly on a reading head. Such position sensors can be implemented with linear grating lines [4], or with focusing diffractive patterns focusing a diffraction order onto a detector [5].
Deformable diffractive structures such as [4] and [5] are well suited for the measurement of the distance between two surfaces, but are unable to detect any lateral displacement, as such displacement would not change the shape of the grooves of the diffractive structure. It is thus an object of the present invention to provide measurements related to lateral movements relative to the diffractive patterns or structures.
US2008/0062432 illustrates a solution detecting a lateral movement between two different gratings. In this case the gratings are used for directing the light in a required direction and are position too far apart to provide any optical interaction.
US2006/007440 illustrates a solution for detecting a relative position between two objects having similar gratings, where the diffracted pattern varies with the relative position. As is shown in FIG. 2 in the document the shape of the distribution of the diffracted light intensity is complex and thus reduces the accuracy of the measurements.
Nanomechanical or near-field grating can be used to detect lateral displacements [9 & 10]. These consist of sets of grating lines situated on two parallel surfaces, forming a multitude of apertures whose width and depth are modified by the lateral displacement of one of the surface relative to the other. These devices consist of grating lines that are smaller than the wavelength of the light used to illuminate the device both in width and thickness. The distance between the two surfaces also has to be smaller than the wavelength of the light. In fact, near-field grating do not produce any diffraction order other than the 0th diffraction order (specular reflection), as this would require the period of the grating to be at least equal to the wavelength of the light. As a consequence, near-field grating must be operated in reflection or transmission, and therefore lack the ability to direct the light at predetermined angles by designing appropriated grating periods. In fact, near-field gratings can be understood as apertures, whose transparency can be tuned by moving two nanostructures relative to each other,